The objective of this program project is to identify and study mechanisms that govern current flow, neural responses and perceptual performance under conditions of intracochlear electrical stimulation. The overall strategy of the research program is reflected in the organization of the five projects, all of which are strongly interrelated. Project I deals with the design, fabrication, and characterization of intracochlear electrodes. An experimental 11-contact array for use Projects III-V will be developed, novel electrode that may provide improved spatial localization of intracochlear current will be designed and characterized, and advanced electrode materials that may carry more electrical charge will be evaluated. Project II deals with the characterization of electrical fields generated by intracochlear electrodes. Finite-element field models of cat and human cochleas will be constructed, and field patterns for clinically- applied and experimental electrodes will be studied. These models will be validated by empirical measurements of current fields in cat cochleas. Project III seeks to identify and study mechanisms governing single-neuron responses to electrical stimuli. Biophysical models of neural response will be developed and evaluated in quantitative studies of single-cell discharge patterns in the auditory nerve and anteroventral cochlear nucleus. Topics to be studied include the effects of changing the spatial geometry of the stimulating current field, correlating neural responses with temporal features of stimulating waveforms, and examining stochastic aspects of neural responses. Project IV deals with the ensemble response of multiple neural elements to electrical stimulation. Models combining the results of field models and single-cell responses will be developed, and their predictions will be evaluated in acute physiological studies that determine neural population responses to electrical signals. Project V investigates psychophysical performance of implanted animals on tasks thought to be encoded at peripheral levels of the auditory system, then conducts physiological studies to record unit activity using the same stimulus paradigms that had been studied behaviorally. Models that attempt to predict perceptual performance from neural responses will be developed and evaluated. All animal studies in Projects III-V will be conducted under conditions of both good and poor spiral ganglion survival. A separate Morphology Core allows all projects to relate findings to details of cochlear anatomy in deafened ears. Likely outcomes of the proposed research include (1) an improved knowledge of patterns of intracochlear current flow and the mechanisms by which electrical current excites auditory neurons, (2) insight into the physiological basis of auditory perceptual performance with cochlear implants, (3) identification of ways to control the spatial extend and temporal fine structure of neural responses, (4) the formulation of biophysically sound approaches to improving speech processors in individual implant patients, and (5) the development of optimized electrode designs for clinical use.